Efficient, reliable, and real-time communication with minimal latency is a critical requirement for public safety organizations and first responders. Toward that end, the Telecommunications Industry Association (TIA) adopted and standardized a suite of Land Mobile Radio (LMR) standards termed Project 25 (P25), as described for example, in the TIA 102-series documents. The P25 standard defines the operation and interfaces of P25-compliant radio systems. Additionally, through a gateway, non-compliant radio systems can also interoperate with a P25 network.
A radio user can obtain service in a coverage area provided by the user's network or radio frequency subsystem (RFSS). To expand a user's coverage area or provide “roaming” service, the industry/TIA currently proposes use of an inter-radio frequency subsystem interface (ISSI) over an Internet Protocol (IP) network, as described for example, in TIA-102.BACA-A published January 2009 by TIA, and any subsequent revisions. Additionally, apart from roaming, the ISSI enables calling between different networks and systems. Generally, the ISSI is used to provide interoperability between different radio and telecommunications networks, regardless of P25 compliance, so long as the networks support an ISSI interface.
The ISSI is also capable of supporting end-to-end protection of media information, or “encryption”. Illustratively, for a receiver to decrypt an incoming encrypted message from a transmitter, the receiver decryption module should be synchronized with the transmitter encryption module. In an ISSI encoded voice message, however, encryption synchronization parameters required for decryption are embedded once at the start of the transmission and in an ongoing manner near the end of each voice superframe due to the nature of the air interface protocol. Furthermore, the encryption synchronization parameters embedded within a given superframe are applicable to the succeeding superframe. Thus, when a user or “subscriber” joins a call later in the synchronization process, the initial encryption synchronization is missed and a late entry condition exists where there is often a significant time delay before the subscriber receives all of the encryption synchronization parameters and is ready to begin decrypting the incoming voice message. Late entry conditions commonly result in significant additional voice truncation for the subscriber.
Accordingly, there is a need for a method of increasing encryption synchronization availability.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments. In addition, the description and drawings do not necessarily require the order illustrated. It will be further appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required.
Apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the various embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Thus, it will be appreciated that for simplicity and clarity of illustration, common and well-understood elements that are useful or necessary in a commercially feasible embodiment may not be depicted in order to facilitate a less obstructed view of these various embodiments.